There has previously been suggested a method for purifying waste water containing chromate ions by passing the waste water through a cation-exchange resin and an anion-exchange resin in that order or, as occasion permits, through an anion-exchange resin alone. According to this method, as the ion-exchange capacity of an ion-exchange resin gradually decreases as the waste water is refined, the resin is subjected to a regeneration treatment to enable reuse. In this method, the regeneration of anion-exchange resin is effected by a conventional technique wherein the anion-exchange resin is subjected to a regeneration treatment while still retaining its ion-exchange capacity to some extent, i.e. immediately after or before the anions entrained in the waste water begin to leak through the anion-exchange resin. When the anion-exchange resin is regenerated as just described, however, the anion-exchange resin still retains adsorbed thereon not only chromate ions originating in the waste water but also other contaminant anions and, therefore, the effluent (eluate) flowing out of the anion-exchange resin inevitably entrains the defiling anions in addition to the chromate ions. If this eluate is recycled to the chromium plating operation, chrome finishing operation, alumite bore-sealing operation, etc., therefore, the contaminants will cause various adverse effects. Thus, the eluate in its unaltered form is not suitable for use as a recycled chromic acid solution. If the chromate ion-containing washing discharged from the chromium plating operation is refined by using an ion-exchange resin and the exhausted anion-exchange resin is regenerated as described above, for example, then the resultant eluate which flows out of the anion-exchange resin entrains therein other contaminating anions in addition to the chromate ions. If the eluate is recycled to the chromium plating operation, the defiling contaminants accumulate in the plating bath and consequently induce defective plating. In one experiment, a bright hard chrome plating treatment was conducted in a Sargent bath containing 250 g/lit. of CrO.sub.3, 2.5 g/lit. of H.sub.2 SO.sub.4 with a 50 A/dm.sup.2 current density and at a 50.degree. C. bath temperature. It was found that adverse effects such as, for example, absence of brightness in the formed plate and degradation of plate hardness appeared when 100 - 200 ppm of nitrate ions were present as contaminants in the bath and that similar phenomena were recognized when 300 to 400 ppm of chlorine ions found their way into the bath.
On the other hand, if the waste water containing chromate ions is released in its untreated form into surface water, it will cause "environmental pollution." Under these circumstances, need has been felt for provision of a method which is capable of refining the waste water containing chromate ions and at the same time capable of recovering from the waste water a chromic acid solution which entrains no contaminating anions and which can be recycled to the chromium plating operation, etc.